1. Field of the Invention
The present invention relates, in general, to a bipolar junction transistor (BJT) formed on silicon carbide and, in particular, to a fully epitaxial vertical SiC bipolar junction transistor with an overgrown base region suitable for use in power microwave applications.
2. Background of the Technology
High power microwave transistors are of great demand in such applications as cellular phone stations, radar systems, etc. Along with silicon lateral MOSFETS, silicon bipolar transistors are now the primary technology used in solid-state radar transmitters. In recent years the advantages of bipolar junction transistors as a power stage in the 0.4 to 4 GHz range have been widely recognized. The newest radar systems are calling for performance requirements that far surpass the capabilities of klystron or tube-type transmitters, but appear ideally suited to solid-state devices.
Today, as the building block power stage unit, a silicon bipolar transistor is the best candidate device for the frequency ranges from UHF through S-band, notwithstanding the advantages in power GaAs and more recent GaN FET technology. Bipolar devices cost-effectively provide for system requirements reliability, ruggedness, electrical performance, packaging, biasing, cooling, availability, and ease of maintenance. Largely due to new developments in processing technology, such as using more shallow emitter diffusions, reduced collector-base time constants, submicron geometries, and more exotic photolithographic processes and etching techniques, creative device packaging, and internal matching techniques, silicon devices are competing effectively up to S-band requirements.
The bipolar junction transistor (BJT) is a well known semiconductor device. A bipolar junction transistor is generally defied as a device formed of a semiconductor material having two p-n junctions in close proximity to one another. In operation, current enters a region (i.e., the emitter) of the device adjacent one of junctions and exits the device from a region (i.e., the collector) of the adjacent the other p-n junction. The collector and emitter have the same conductivity type (i.e., either p or n). A portion of semiconductor material having the opposite conductivity type from the collector and the emitter is positioned between the collector and the emitter. This material is known as the base. The two p-n junctions of the transistor are formed where the collector meets the base and where the base meets the emitter. Because of their respective structures and conductivity types, bipolar junction transistors are generally referred to as either n-p-n or p-n-p transistors.
In operation, when current is injected into or extracted from the base (depending upon whether the transistor is n-p-n or p-n-p), the flow of charge carriers (i.e. electrons or holes) which can move from the collector to the emitter will be effected. Typically, small currents applied to the base can control proportionally large currents passing through the transistor, making the bipolar junction transistor useful as a component of electronic circuits.
Silicon carbide has known advantageous characteristics as a semiconductor material. These characteristics include a wide bandgap, a high thermal conductivity, a high melting point, a high electric field breakdown strength, a low dielectric constant, and a high saturated electron drift velocity. As a result, electronic devices formed from silicon carbide should have the capability of operating at higher temperatures, at higher device densities, at higher speeds, at higher power levels and even under higher levels of radiation than other semiconductor materials. Silicon carbide bipolar transistors, which have excellent blocking capability, small specific on-resistance, and high thermal conductivity, are therefore promising candidates to replace silicon devices, particularly in power transistors for high frequency applications.
Silicon carbide bipolar junction transistors are known. See, for example, v. Mxc3xcnch et al., xe2x80x9cSilicon Carbide Bipolar Transistorxe2x80x9d, Solid State Electronics, Vol.21, pp. 479-480 (1978); Luo et al., xe2x80x9cDemonstration of 4H-SiC Power Bipolar Junction Transistorsxe2x80x9d, Electronic Letters, Vol. 36, No. 17 (2000); Tang et al., xe2x80x9cAn Implanted-Emitter 4H-SiC Bipolar Transistor with High Current Gainxe2x80x9d, IEEE Electron Device Letters, Volume 22, Issue 3, pp. 119-120 (2001) and U.S. Pat. Nos. 4,762,806, 4,945,394 and 6,218,254. A 4H-SiC bipolar junction transistor, for example, has been reported to demonstrate a blocking voltage of 1.8 kV, on resistance of 10.8 mxcexa9xc2x7cm2, and a temperature stable current gain with a peak value of 20. See Agarwal et al., xe2x80x9cDevelopment of Silicon Carbide High Temperature Bipolar Devicesxe2x80x9d, HITEC, Albuquerque, N.M ex. (2000). This SiC transistor also showed a positive temperature coefficient in the on-resistance characteristics, which may facilitate paralleling the device. These properties could confer advantages over silicon bipolar junction transistors, where thermal runaway can be a problem.
At high frequencies, the operating characteristics of silicon carbide bipolar junction transistors are highly dependent on the thickness of the p-base layer. Generally, thinner p-base layers confer better high frequency performance. However, it can be difficult to form base layers having desirable thicknesses for high frequency (e.g., microwave) applications. Further, achieving adequate ohmic contact to a very thin base region while minimizing peripheral base resistance can be difficult. As a solution to this problem, v. Mxc3xcnch et al., supra, proposed thinning an epitaxially grown SiC base layer and forming an overgrown emitter layer thereon.
The lateral dimensions of a bipolar junction transistor can also affect the high frequency performance of the device. It is generally desirable to shrink or scale down the dimensions of the device. Features are typically formed in semiconductor devices using photolithography techniques. Such techniques, however, require numerous process steps and can be costly to implement. Further, the resolutions obtainable using conventional photolithography techniques are limited. Self alignment techniques have also been proposed as an alternative to photolithographic techniques. See, for example, U.S. Pat. No. 6,218,254 (hereinafter the ""254 patent). Self alignment techniques are manufacturing techniques wherein device features automatically and inherently align as a result of the manufacturing process. The use of self-alignment techniques can allow for the formation of fine features while simplifying the manufacture of the device. The ""254 patent discloses a method of fabrication of SiC bipolar junction transistors having self-aligned ion implanted n-plus emitter regions or ion implanted p-plus regions for base ohmic contacts. The high energies required for ion implantation, however, can result in damage to the device. Further, ion implantation typically requires a high temperature annealing step to activate the implanted impurities. These additional process steps add to both the cost and complexity of the manufacturing process.
There still remains a need, therefore, for improved methods of making SiC bipolar junction transistors having sufficiently thin p-base regions for improved high frequency performance. Such methods would ideally allow for adequate ohmic contact to the base and would provide devices having minimal peripheral base resistance.
According to a first aspect of the invention, a method of making a SiC bipolar junction transistor is provided. The method includes steps of: providing a collector comprising SiC doped with a donor material, the collector having first and second major surfaces; optionally forming a drift layer on the first major surface of the collector, the drift layer comprising SiC doped with a donor material; forming a first base layer on the first major surface of the collector or on the drift layer, the first base layer comprising SiC doped with an acceptor material; forming an emitter layer on the first base layer, the emitter layer comprising SiC doped with a donor material; etching through the emitter layer and the first base layer to expose the collector or drift layer and to form one or more raised emitter regions having upper surfaces defined by regions of etched emitter and first base layer, the etched regions comprising bottom and sidewall surfaces; forming a second base layer comprising SiC doped with an acceptor material, wherein the second base layer covers the bottom and sidewall surfaces of the etched regions and the upper surfaces of emitter regions; forming emitter contact openings through the second base layer on upper surfaces of the emitter regions to expose emitter material, the emitter contact openings having bottom and sidewall surfaces; forming base contacts on surfaces of the second base material in the etched regions; and forming emitter contacts on surfaces of the emitter material in the emitter contact openings.
According to a second aspect of the invention, a silicon carbide bipolar junction transistor is also provided. The transistor includes a collector having first and second major surfaces. The collector comprises silicon carbide doped with a donor material. A drift layer can be formed on the first major surface of the collector. The drift layer comprises silicon carbide doped with a donor material. A plurality of raised emitter regions are formed on either the drift layer or on the first major surface of the collector. The raised emitter regions are non-coextensive with the drift layer or the collector and are formed on a central portion of the drift layer or collector. The raised emitter regions include an upper emitter layer formed on a lower base layer. The lower base layer is in contact with either the collector or drift layer and the upper emitter layer is substantially coextensive with the lower base layer. The upper emitter layer comprises SiC doped with a donor material and the lower base layer comprises SiC doped with an acceptor material. The transistor also includes a second base layer doped with an acceptor material at a higher doping level than the lower base layer. The second base layer is formed on exposed surfaces of the collector or drift layer and on exposed surfaces of the raised emitter regions. Emitter openings can be formed in the second base layer on upper surfaces of the emitter regions and emitter contacts can be disposed in the emitter openings. Base contacts can be formed on the second base layer. According to a preferred embodiment of the invention, the first base layer of the transistor has a thickness of 0.2 xcexcm or less.
According to a further embodiment of the invention, the silicon carbide bipolar junction transistor further includes a base contact layer formed on and coextensive with the second base layer. Emitter contact openings can be formed in the base contact layer and the underlying second base layer on upper surfaces of the emitter regions such that the emitter contact openings in the second base layer are substantially aligned with the emitter contact openings in the base contact layer. Emitter contacts can be disposed on exposed emitter material in the emitter contact openings.